1. Field of the Invention
The present invention relates to low viscosity, concentrated water soluble organic salts of the lanthanides; methods for their preparations; and use. More particularly, this invention relates to the preparation of low viscosity, concentrated aqueous solutions of organic salts of cerium which have particular utility in the preparation of pollution reduction catalysts.
2. Description of the Prior Art
Multilayer catalysts that have been universally accepted in the catalytic converters of the automobile industry convert unburned hydrocarbons and carbon monoxide to carbon dioxide and water and additionally convert oxides of nitrogen to elemental nitrogen. These reactions occur at elevated temperatures, generally in the range of from about 100.degree. C. to about 1300.degree. C. The five major components of these vehicle exhaust catalyst usually comprise the substrate, the undercoat, stabilizers, metal promoters and platinum group metals. The support is primarily a low surface area structure which serves as a catalyst foundation by providing high rigidity and high strength. In the automotive industry, the support is generally a dense, honeycomb type structure with a thin, nonporous surface usually fabricated from cordierite. To this low surface area support is applied an undercoat having relatively high surface area which serves as an anchor or base for the catalytic entity forming the outermost surface of the catalytic composite.
Alternatively, the catalyst undercoat material per se may be compressed or extruded into various geometric forms and used directly to support the catalyst entity. Such a solid may be in a variety of forms including powders, granules, sheets, spheres, extradites, honeycombs, or monolith structures, cylinders or rings, saddle stars and the like. The highly porous undercoat is usually comprised of alumina. Rare earth oxide stabilizers, such as the lanthanide oxides, especially the cerium oxides, are often admixed with the alumina undercoat to maintain the high surface area at the elevated temperatures, such as 1000.degree. C., that are often encountered during use.
U.S. Pat. No. 4,996,031 discloses a catalyst undercoat material comprising alumina and at least one lanthanide oxide which are co-precipitated from soluble salts. The lanthanide oxide is taught to be present in an amount of from about 1 to about 45 weight percent based on the total weight of the oxide and alumina present. Suitable aluminum salts disclosed include the nitrate, sulfate, chloride, acetate and oxalate. Suitable water soluble lanthanide salts disclosed include cerium nitrate, sulfate, chloride, acetate, lactate, propionate and butyrate; lanthanum nitrate, acetate and sulfate; praseodymium nitrate; and neodymium nitrate. It was noted that the nitrates were preferred because some of the other anions such as the sulfate can appear as an occluded ion in the precipitate, resulting in catalyst poisoning.
U.S. Pat. No. 4,868,149 discloses the use of a lanthanum laced cerium oxide/alumina slurry as a catalytic undercoat. One disclosed method for preparing the slurry involves impregnating the alumina with an aqueous solution containing a water soluble cerium compound and calcining the mixture in air at a temperature of from about 400.degree. to 700.degree. C. to realized well dispersed cerium oxide on the alumina. Another slurry preparation method disclosed involves co-precipitating or co-gelling a cerium compound with an aluminum compound and pyrolyzing same to achieve 5 to 50% by weight cerium oxide on the alumina.
Onto and into the porous undercoat is applied by impregnation, immersion, spraying or other means, the catalytic coating comprising oxidation catalysts from the precious metal groups and promoters.
These catalytic ingredients are well known in the art. Useful for this surface catalytic coating are certain of the base oxides of elements and mixtures thereof which are identified in Group 8 of the Periodic Table and which include the precious metals and individual elements such as manganese, chromium, and copper as their oxides or interreaction products. Particularly useful as surface catalytic components in the automobile emission reduction art are the precious metals, particularly platinum, palladium and rhodium.
Promoters, such as the lanthanide oxides, particularly cerium oxide (ceria) often provide dramatic increases in the catalytic efficiency of the precious metal catalysts under certain emission reduction conditions. For example, rhodium is extremely sensitive to deactivation at high temperatures under the lean operating conditions which can be encountered during extensive high speed driving. This deactivation is thought to be due to a strong rhodium-alumina interaction, which fixes rhodium in a high oxidation state which is difficult to reduce. This interaction can be reduced by the incorporation of ceria into the catalyst. Additionally, in the presence of water, significant increases in carbon monoxide conversion can be realized over precious metal catalysts in the presence of a cerium oxide additive.
U.S. Pat. No. 3,993,572 discloses catalyst compositions containing a platinum group, a rare earth metal oxide such as the cerium, samarium and praseodymium oxides, and an alumina component. The catalyst compositions may be prepared by co-precipitating the ingredients or by impregnating the alumina powder with, e.g., cerium salts.
U.S. Pat. Nos. 3,867,312 and 3,899,444 disclose another procedure which includes preparing an aqueous solution of water soluble decomposable salts of a rare earth metal and a water soluble aluminum salt, evaporating the free water from the solution and then heating the resultant mixture. A uniform mixture of salts is obtained; subsequently decomposed; and then cooled to produce a self-supporting catalytic entity.
The aforementioned catalysts are also used in the chemical industry or in abatement processes for the disposal of combustible or toxic materials including reducing pollutants in waste gases.
The above discussion and cited patents are generally representative of the state of the art. From these teachings, it can be seen that the water soluble organic salts of the lanthanides, especially the water soluble cerium salts are highly useful in the catalytic art. However, while the cerium acetate, propionate and lactate salts do not realize undesirable occludable anions that can negatively affect the activity of the final compositions nor produce environmentally unacceptable by-products under pyrolysis conditions, they have relatively low water solubilities. This low solubility in water results in the necessity of multiple steps to achieve the high lanthanide oxide loadings required by the above described catalytic art.
Furthermore, not only is it desirable to utilize water soluble organic lanthanide salts at high concentrations, but the concentrated solutions themselves must, of necessity, be of low viscosity to enable the solution to easily coat particles of the other catalyst components and/or to easily penetrate into the interstices such as the honeycomb structures, of the support or undercoat. Merely diluting the solutions to lower the viscosity is unacceptable for, it will also reduce the much sought after high lanthanide concentration of the solution.